Temporal correction of tone scale errors in a digital printer

ABSTRACT

A method for correcting tone-level non-uniformities in a digital printing system includes printing a test target having a set of uniform test patches. The printed test target is automatically analyzed to determine tone-level errors as a function of cross-track position for each of the test patches. A tone-level correction function is determined and represented using a set of one-dimensional feature vectors which specifies tone-level corrections as a function of cross-track position, pixel value and time. Corrected image data is determined by using the tone-level correction function to determine a tone-level correction value for each image pixel responsive to the input pixel value, cross-track position and time. The corrected image data is printed using the digital printing system to provide a printed image with reduced tone-level errors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 63/104,029, filed Oct. 22, 2020, which is incorporatedherein by reference in its entirety.

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. 16/417,731, entitled “Correcting cross-track errorsin a linear printhead”, by Kuo et al.; to commonly assigned, co-pendingU.S. patent application Ser. No. 16/417,763, entitled “Printer withcross-track position error correction”, by Kuo et al.; to commonlyassigned, co-pending U.S. patent application Ser. No. 16/564,235,entitled: “Correcting-in-track errors in a linear printhead”, by Kuo etal.; to commonly assigned, co-pending U.S. patent application Ser. No.16/564,255, entitled: “Printer with in-track position error correction”,by Kuo et al.; and to commonly assigned, co-pending U.S. patentapplication Ser. No. 16/581,909, entitled “Correcting tone scale errorsin a digital printer”, each of which is incorporated herein byreference.

FIELD OF THE INVENTION

This invention pertains to the field of digital printing, and moreparticularly to a method for correcting tone scale errors that vary withtime and position.

BACKGROUND OF THE INVENTION

Electrophotography is a useful process for printing images on a receiver(or “imaging substrate”), such as a piece or sheet of paper or anotherplanar medium (e.g., glass, fabric, metal, or other objects) as will bedescribed below. In this process, an electrostatic latent image isformed on a photoreceptor by uniformly charging the photoreceptor andthen discharging selected areas of the uniform charge to yield anelectrostatic charge pattern corresponding to the desired image (i.e., a“latent image”).

After the latent image is formed, charged toner particles are broughtinto the vicinity of the photoreceptor and are attracted to the latentimage to develop the latent image into a toner image. Note that thetoner image may not be visible to the naked eye depending on thecomposition of the toner particles (e.g., clear toner).

After the latent image is developed into a toner image on thephotoreceptor, a suitable receiver is brought into juxtaposition withthe toner image. A suitable electric field is applied to transfer thetoner particles of the toner image to the receiver to form the desiredprint image on the receiver. The imaging process is typically repeatedmany times with reusable photoreceptors.

The receiver is then removed from its operative association with thephotoreceptor and subjected to heat or pressure to permanently fix(i.e., “fuse”) the print image to the receiver. Plural print images(e.g., separation images of different colors) can be overlaid on thereceiver before fusing to form a multi-color print image on thereceiver.

It has been observed that there can be uniformity artifacts such asimage streaks in printed images formed using digital printing systemssuch as electrophotographic printers. In many cases, the characteristicsof these uniformity artifacts have been found to vary as a function oftime and image density. There remains a need for improved methods tocharacterize and correct for such artifacts.

SUMMARY OF THE INVENTION

The present invention represents a method for correcting tone-levelnon-uniformities in a digital printing system, including:

a) providing digital image data for a test target including a set ofuniform test patches, each test patch having an associated pixel valueand extending in a cross-track direction;

b) printing the test target using the digital printing system to providea printed test target;

c) using a digital image capture system to capture an image of theprinted test target;

d) using a data processing system to automatically analyze the capturedimage to determine a measured tone-level as a function of cross-trackposition for each of the test patches;

e) comparing the measured tone-levels to nominal tone-levels for eachtest patch to determine measured tone-level errors as a function ofcross-track position for each of the test patches;

f) determining a tone-level correction function responsive to themeasured tone-level errors, wherein the tone-level correction functionspecifies tone-level corrections to be applied as a function ofcross-track position, pixel value and time;

g) storing a representation of the tone-level correction function in adigital memory, wherein the representation of the tone-level correctionfunction includes a set of one-dimensional feature vectors correspondingto a decomposition of the tone-level correction function into asummation of outer vector products of the one-dimensional featurevectors;

h) receiving digital image data for an input digital image to be printedby the digital imaging system, wherein the digital image data specifiesinput pixel values for an array of image pixels;

i) determining corrected image data by:

-   -   using the stored representation of the tone-level correction        function to determine a tone-level correction value for each        image pixel responsive to the input pixel value of the image        pixel and the cross-track position of the image pixel; and    -   modifying the input pixel value for each image pixel responsive        to the determined tone-level correction value to provide a        corrected pixel value;

j) printing the corrected image data using the digital printing systemto provide a printed image with reduced tone-level errors;

wherein the decomposition of the tone-level correction function has theform:

${\Delta_{c}( {x,v,t} )} \cong {\sum\limits_{f = 1}^{n}{{i_{f}(x)}o\;{k_{f}(v)}{{ol}_{f}(t)}}}$wherein Δ_(c)(x,v,t) is the tone-level correction function, i_(f)(x) areone-dimensional cross-track feature vectors taken along the cross-trackaxis of the tone-level correction function, k_(f)(v) are one-dimensionalpixel value feature vectors taken along the pixel value axis of thetone-level correction function, l_(f)(t) are one-dimensional temporalfeature vectors taken along the time axis of the tone-level correctionfunction, x is the cross-track position, v is the pixel value, t is thetime value, f is a feature vector index, n is the total number offeature vectors, and ∘ is the outer vector product operator.

This invention has the advantage that tone-level errors such as streakartifacts that vary as a function of cross-track position, tone-leveland time can be significantly reduced, regardless of the source of thoseerrors.

It has the additional advantage that the representation of thetone-level correction function using the one-dimensional feature vectorsrequires a reduced amount of storage memory.

It has the further advantage that the tone-level correction function canalso be used to correct for tone-level errors that vary as a function ofin-track position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross-section of an electrophotographic printersuitable for use with various embodiments;

FIG. 2 is an elevational cross-section of one printing subsystem of theelectrophotographic printer of FIG. 1;

FIG. 3 shows a conventional processing path for producing a printedimage using a pre-processing system couple to a print engine;

FIG. 4 shows a processing path including a print engine that is adaptedto produce printed images from image data supplied by a variety ofdifferent pre-processing systems;

FIG. 5 shows additional details for the resolution modificationprocessor and the halftone processor of FIG. 4;

FIG. 6 shows a flow chart for a computational halftoning process thatcan be used for the halftoning operation of FIG. 5;

FIG. 7 illustrates a dot shape parameter function useful for thecomputational halftoning process of FIG. 6;

FIG. 8 illustrates a threshold value function useful for thecomputational halftoning process of FIG. 6;

FIG. 9 illustrates an edge softness parameter function useful for thecomputational halftoning process of FIG. 6;

FIG. 10 illustrates example halftoned images formed using thecomputational halftoning process of FIG. 6;

FIG. 11 shows an improved processing path including a print engine thatis adapted to apply tone-level corrections that are a function ofcross-track position and pixel value;

FIG. 12 shows additional details for the resolution/tone-level processorand the halftone processor of FIG. 11;

FIG. 13 shows a flowchart of a method for determining a tone-levelcorrection function in accordance with an exemplary embodiment;

FIG. 14 illustrates an exemplary test target that includes uniform testpatches useful for determining a tone-level correction function;

FIG. 15 is a graph showing exemplary tone-level measurements as afunction of cross-track position for a set of uniform test patches;

FIG. 16 is a graph showing exemplary tone-level errors as a function ofcross-track position for a set of uniform test patches;

FIG. 17 is a graph of an exemplary scanner calibration function whichrelates scanner code values to corresponding input code values.

FIG. 18 illustrates an exemplary tone-level correction function;

FIG. 19 illustrates an exemplary set of vectors determined to representthe tone-level correction function of FIG. 18;

FIG. 20 shows a flowchart of a method for determining modified imagepixels in accordance with an exemplary embodiment; and

FIG. 21 is a high-level diagram showing the components of a system forprocessing images in accordance with the present invention.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.Identical reference numerals have been used, where possible, todesignate identical features that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated, or as are readily apparent to one of skill in the art. Theuse of singular or plural in referring to the “method” or “methods” andthe like is not limiting. It should be noted that, unless otherwiseexplicitly noted or required by context, the word “or” is used in thisdisclosure in a non-exclusive sense.

As used herein, the terms “parallel” and “perpendicular” have atolerance of ±10°.

As used herein, “sheet” is a discrete piece of media, such as receivermedia for an electrophotographic printer (described below). Sheets havea length and a width. Sheets are folded along fold axes (e.g.,positioned in the center of the sheet in the length dimension, andextending the full width of the sheet). The folded sheet contains two“leaves,” each leaf being that portion of the sheet on one side of thefold axis. The two sides of each leaf are referred to as “pages.” “Face”refers to one side of the sheet, whether before or after folding.

As used herein, “toner particles” are particles of one or morematerial(s) that are transferred by an electrophotographic (EP) printerto a receiver to produce a desired effect or structure (e.g., a printimage, texture, pattern, or coating) on the receiver. Toner particlescan be ground from larger solids, or chemically prepared (e.g.,precipitated from a solution of a pigment and a dispersant using anorganic solvent), as is known in the art. Toner particles can have arange of diameters (e.g., less than 8 μm, on the order of 10-15 μm, upto approximately 30 μm, or larger), where “diameter” preferably refersto the volume-weighted median diameter, as determined by a device suchas a Coulter Multisizer. When practicing this invention, it ispreferable to use larger toner particles (i.e., those having diametersof at least 20 μm) in order to obtain the desirable toner stack heightsthat would enable macroscopic toner relief structures to be formed.

“Toner” refers to a material or mixture that contains toner particles,and that can be used to form an image, pattern, or coating whendeposited on an imaging member including a photoreceptor, aphotoconductor, or an electrostatically-charged or magnetic surface.Toner can be transferred from the imaging member to a receiver. Toner isalso referred to in the art as marking particles, dry ink, or developer,but note that herein “developer” is used differently, as describedbelow. Toner can be a dry mixture of particles or a suspension ofparticles in a liquid toner base.

As mentioned already, toner includes toner particles; it can alsoinclude other types of particles. The particles in toner can be ofvarious types and have various properties. Such properties can includeabsorption of incident electromagnetic radiation (e.g., particlescontaining colorants such as dyes or pigments), absorption of moistureor gasses (e.g., desiccants or getters), suppression of bacterial growth(e.g., biocides, particularly useful in liquid-toner systems), adhesionto the receiver (e.g., binders), electrical conductivity or low magneticreluctance (e.g., metal particles), electrical resistivity, texture,gloss, magnetic remanence, florescence, resistance to etchants, andother properties of additives known in the art.

In single-component or mono-component development systems, “developer”refers to toner alone. In these systems, none, some, or all of theparticles in the toner can themselves be magnetic. However, developer ina mono-component system does not include magnetic carrier particles. Indual-component, two-component, or multi-component development systems,“developer” refers to a mixture including toner particles and magneticcarrier particles, which can be electrically-conductive or-non-conductive. Toner particles can be magnetic or non-magnetic. Thecarrier particles can be larger than the toner particles (e.g., 15-20 μmor 20-300 μm in diameter). A magnetic field is used to move thedeveloper in these systems by exerting a force on the magnetic carrierparticles. The developer is moved into proximity with an imaging memberor transfer member by the magnetic field, and the toner or tonerparticles in the developer are transferred from the developer to themember by an electric field, as will be described further below. Themagnetic carrier particles are not intentionally deposited on the memberby action of the electric field; only the toner is intentionallydeposited. However, magnetic carrier particles, and other particles inthe toner or developer, can be unintentionally transferred to an imagingmember. Developer can include other additives known in the art, such asthose listed above for toner. Toner and carrier particles can besubstantially spherical or non-spherical.

The electrophotographic process can be embodied in devices includingprinters, copiers, scanners, and facsimiles, and analog or digitaldevices, all of which are referred to herein as “printers.” Variousembodiments described herein are useful with electrostatographicprinters such as electrophotographic printers that employ tonerdeveloped on an electrophotographic receiver, and ionographic printersand copiers that do not rely upon an electrophotographic receiver.Electrophotography and ionography are types of electrostatography(printing using electrostatic fields), which is a subset ofelectrography (printing using electric fields). The present inventioncan be practiced using any type of electrographic printing system,including electrophotographic and ionographic printers.

A digital reproduction printing system (“printer”) typically includes adigital front-end processor (DFE), a print engine (also referred to inthe art as a “marking engine”) for applying toner to the receiver, andone or more post-printing finishing system(s) (e.g., a UV coatingsystem, a glosser system, or a laminator system). A printer canreproduce pleasing black-and-white or color images onto a receiver. Aprinter can also produce selected patterns of toner on a receiver, whichpatterns (e.g., surface textures) do not correspond directly to avisible image.

In an embodiment of an electrophotographic modular printing machineuseful with various embodiments (e.g., the NEXPRESS SX 3900 printermanufactured by Eastman Kodak Company of Rochester, N.Y.) color-tonerprint images are made in a plurality of color imaging modules arrangedin tandem, and the print images are successively electrostaticallytransferred to a receiver adhered to a transport web moving through themodules. Colored toners include colorants, (e.g., dyes or pigments)which absorb specific wavelengths of visible light. Commercial machinesof this type typically employ intermediate transfer members in therespective modules for transferring visible images from thephotoreceptor and transferring print images to the receiver. In otherelectrophotographic printers, each visible image is directly transferredto a receiver to form the corresponding print image.

Electrophotographic printers having the capability to also deposit cleartoner using an additional imaging module are also known. The provisionof a clear-toner overcoat to a color print is desirable for providingfeatures such as protecting the print from fingerprints, reducingcertain visual artifacts or providing desired texture or surface finishcharacteristics. Clear toner uses particles that are similar to thetoner particles of the color development stations but without coloredmaterial (e.g., dye or pigment) incorporated into the toner particles.However, a clear-toner overcoat can add cost and reduce color gamut ofthe print; thus, it is desirable to provide for operator/user selectionto determine whether or not a clear-toner overcoat will be applied tothe entire print. A uniform layer of clear toner can be provided. Alayer that varies inversely according to heights of the toner stacks canalso be used to establish level toner stack heights. The respectivecolor toners are deposited one upon the other at respective locations onthe receiver and the height of a respective color toner stack is the sumof the toner heights of each respective color. Uniform stack heightprovides the print with a more even or uniform gloss.

FIGS. 1-2 are elevational cross-sections showing portions of a typicalelectrophotographic printer 100 useful with various embodiments. Printer100 is adapted to produce images, such as single-color images (i.e.,monochrome images), or multicolor images such as CMYK, or pentachrome(five-color) images, on a receiver. Multicolor images are also known as“multi-component” images. One embodiment involves printing using anelectrophotographic print engine having five sets of single-colorimage-producing or image-printing stations or modules arranged intandem, but more or less than five colors can be combined on a singlereceiver. Other electrophotographic writers or printer apparatus canalso be included. Various components of printer 100 are shown asrollers; other configurations are also possible, including belts.

Referring to FIG. 1, printer 100 is an electrophotographic printingapparatus having a number of tandemly-arranged electrophotographicimage-forming printing subsystems 31, 32, 33, 34, 35, also known aselectrophotographic imaging subsystems. Each printing subsystem 31, 32,33, 34, 35 produces a single-color toner image for transfer using arespective transfer subsystem 50 (for clarity, only one is labeled) to areceiver 42 successively moved through the modules. In some embodimentsone or more of the printing subsystem 31, 32, 33, 34, 35 can print acolorless toner image, which can be used to provide a protectiveovercoat or tactile image features. Receiver 42 is transported fromsupply unit 40, which can include active feeding subsystems as known inthe art, into printer 100 using a transport web 81. In variousembodiments, the visible image can be transferred directly from animaging roller to a receiver, or from an imaging roller to one or moretransfer roller(s) or belt(s) in sequence in transfer subsystem 50, andthen to receiver 42. Receiver 42 is, for example, a selected section ofa web or a cut sheet of a planar receiver media such as paper ortransparency film.

In the illustrated embodiments, each receiver 42 can have up to fivesingle-color toner images transferred in registration thereon during asingle pass through the five printing subsystems 31, 32, 33, 34, 35 toform a pentachrome image. As used herein, the term “pentachrome” impliesthat in a print image, combinations of various of the five colors arecombined to form other colors on the receiver at various locations onthe receiver, and that all five colors participate to form processcolors in at least some of the subsets. That is, each of the five colorsof toner can be combined with toner of one or more of the other colorsat a particular location on the receiver to form a color different thanthe colors of the toners combined at that location. In an exemplaryembodiment, printing subsystem 31 forms black (K) print images, printingsubsystem 32 forms yellow (Y) print images, printing subsystem 33 formsmagenta (M) print images, and printing subsystem 34 forms cyan (C) printimages.

Printing subsystem 35 can form a red, blue, green, or other fifth printimage, including an image formed from a clear toner (e.g., one lackingpigment). The four subtractive primary colors, cyan, magenta, yellow,and black, can be combined in various combinations of subsets thereof toform a representative spectrum of colors. The color gamut of a printer(i.e., the range of colors that can be produced by the printer) isdependent upon the materials used and the process used for forming thecolors. The fifth color can therefore be added to improve the colorgamut. In addition to adding to the color gamut, the fifth color canalso be a specialty color toner or spot color, such as for makingproprietary logos or colors that cannot be produced with only CMYKcolors (e.g., metallic, fluorescent, or pearlescent colors), or a cleartoner or tinted toner. Tinted toners absorb less light than theytransmit, but do contain pigments or dyes that move the hue of lightpassing through them towards the hue of the tint. For example, ablue-tinted toner coated on white paper will cause the white paper toappear light blue when viewed under white light, and will cause yellowsprinted under the blue-tinted toner to appear slightly greenish underwhite light.

Receiver 42 a is shown after passing through printing subsystem 31.Print image 38 on receiver 42 a includes unfused toner particles.Subsequent to transfer of the respective print images, overlaid inregistration, one from each of the respective printing subsystems 31,32, 33, 34, 35, receiver 42 a is advanced to a fuser module 60 (i.e., afusing or fixing assembly) to fuse the print image 38 to the receiver 42a. Transport web 81 transports the print-image-carrying receivers to thefuser module 60, which fixes the toner particles to the respectivereceivers, generally by the application of heat and pressure. Thereceivers are serially de-tacked from the transport web 81 to permitthem to feed cleanly into the fuser module 60. The transport web 81 isthen reconditioned for reuse at cleaning station 86 by cleaning andneutralizing the charges on the opposed surfaces of the transport web81. A mechanical cleaning station (not shown) for scraping or vacuumingtoner off transport web 81 can also be used independently or withcleaning station 86. The mechanical cleaning station can be disposedalong the transport web 81 before or after cleaning station 86 in thedirection of rotation of transport web 81.

In the illustrated embodiment, the fuser module 60 includes a heatedfusing roller 62 and an opposing pressure roller 64 that form a fusingnip 66 therebetween. In an embodiment, fuser module 60 also includes arelease fluid application substation 68 that applies release fluid,e.g., silicone oil, to fusing roller 62. Alternatively, wax-containingtoner can be used without applying release fluid to the fusing roller62. Other embodiments of fusers, both contact and non-contact, can beemployed. For example, solvent fixing uses solvents to soften the tonerparticles so they bond with the receiver. Photoflash fusing uses shortbursts of high-frequency electromagnetic radiation (e.g., ultravioletlight) to melt the toner. Radiant fixing uses lower-frequencyelectromagnetic radiation (e.g., infrared light) to more slowly melt thetoner. Microwave fixing uses electromagnetic radiation in the microwaverange to heat the receivers (primarily), thereby causing the tonerparticles to melt by heat conduction, so that the toner is fixed to thereceiver.

The fused receivers (e.g., receiver 42 b carrying fused image 39) aretransported in series from the fuser module 60 along a path either to anoutput tray 69, or back to printing subsystems 31, 32, 33, 34, 35 toform an image on the backside of the receiver (i.e., to form a duplexprint). Receivers 42 b can also be transported to any suitable outputaccessory. For example, an auxiliary fuser or glossing assembly canprovide a clear-toner overcoat. Printer 100 can also include multiplefuser modules 60 to support applications such as overprinting, as knownin the art.

In various embodiments, between the fuser module 60 and the output tray69, receiver 42 b passes through a finisher 70. Finisher 70 performsvarious paper-handling operations, such as folding, stapling,saddle-stitching, collating, and binding.

Printer 100 includes main printer apparatus logic and control unit (LCU)99, which receives input signals from various sensors associated withprinter 100 and sends control signals to various components of printer100. LCU 99 can include a microprocessor incorporating suitable look-uptables and control software executable by the LCU 99. It can alsoinclude a field-programmable gate array (FPGA), programmable logicdevice (PLD), programmable logic controller (PLC) (with a program in,e.g., ladder logic), microcontroller, or other digital control system.LCU 99 can include memory for storing control software and data. In someembodiments, sensors associated with the fuser module 60 provideappropriate signals to the LCU 99. In response to the sensor signals,the LCU 99 issues command and control signals that adjust the heat orpressure within fusing nip 66 and other operating parameters of fusermodule 60. This permits printer 100 to print on receivers of variousthicknesses and surface finishes, such as glossy or matte.

FIG. 2 shows additional details of printing subsystem 31, which isrepresentative of printing subsystems 32, 33, 34, and 35 (FIG. 1).Photoreceptor 206 of imaging member 111 includes a photoconductive layerformed on an electrically conductive substrate. The photoconductivelayer is an insulator in the substantial absence of light so thatelectric charges are retained on its surface. Upon exposure to light,the charge is dissipated. In various embodiments, photoreceptor 206 ispart of, or disposed over, the surface of imaging member 111, which canbe a plate, drum, or belt. Photoreceptors can include a homogeneouslayer of a single material such as vitreous selenium or a compositelayer containing a photoconductor and another material. Photoreceptors206 can also contain multiple layers.

Charging subsystem 210 applies a uniform electrostatic charge tophotoreceptor 206 of imaging member 111. In an exemplary embodiment,charging subsystem 210 includes a wire grid 213 having a selectedvoltage. Additional necessary components provided for control can beassembled about the various process elements of the respective printingsubsystems. Meter 211 measures the uniform electrostatic charge providedby charging subsystem 210.

An exposure subsystem 220 is provided for selectively modulating theuniform electrostatic charge on photoreceptor 206 in an image-wisefashion by exposing photoreceptor 206 to electromagnetic radiation toform a latent electrostatic image. The uniformly-charged photoreceptor206 is typically exposed to actinic radiation provided by selectivelyactivating particular light sources in an LED array or a laser deviceoutputting light directed onto photoreceptor 206. In embodiments usinglaser devices, a rotating polygon (not shown) is sometimes used to scanone or more laser beam(s) across the photoreceptor in the fast-scandirection. One pixel site is exposed at a time, and the intensity orduty cycle of the laser beam is varied at each dot site. In embodimentsusing an LED array, the array can include a plurality of LEDs arrangednext to each other in a line, all dot sites in one row of dot sites onthe photoreceptor can be selectively exposed simultaneously, and theintensity or duty cycle of each LED can be varied within a line exposuretime to expose each pixel site in the row during that line exposuretime.

As used herein, an “engine pixel” is the smallest addressable unit onphotoreceptor 206 which the exposure subsystem 220 (e.g., the laser orthe LED) can expose with a selected exposure different from the exposureof another engine pixel. Engine pixels can overlap (e.g., to increaseaddressability in the slow-scan direction). Each engine pixel has acorresponding engine pixel location, and the exposure applied to theengine pixel location is described by an engine pixel level.

The exposure subsystem 220 can be a write-white or write-black system.In a write-white or “charged-area-development” system, the exposuredissipates charge on areas of photoreceptor 206 to which toner shouldnot adhere. Toner particles are charged to be attracted to the chargeremaining on photoreceptor 206. The exposed areas therefore correspondto white areas of a printed page. In a write-black or “discharged-areadevelopment” system, the toner is charged to be attracted to a biasvoltage applied to photoreceptor 206 and repelled from the charge onphotoreceptor 206. Therefore, toner adheres to areas where the charge onphotoreceptor 206 has been dissipated by exposure. The exposed areastherefore correspond to black areas of a printed page.

In the illustrated embodiment, meter 212 is provided to measure thepost-exposure surface potential within a patch area of a latent imageformed from time to time in a non-image area on photoreceptor 206. Othermeters and components can also be included (not shown).

A development station 225 includes toning shell 226, which can berotating or stationary, for applying toner of a selected color to thelatent image on photoreceptor 206 to produce a developed image onphotoreceptor 206 corresponding to the color of toner deposited at thisprinting subsystem 31. Development station 225 is electrically biased bya suitable respective voltage to develop the respective latent image,which voltage can be supplied by a power supply (not shown). Developeris provided to toning shell 226 by a supply system (not shown) such as asupply roller, auger, or belt. Toner is transferred by electrostaticforces from development station 225 to photoreceptor 206. These forcescan include Coulombic forces between charged toner particles and thecharged electrostatic latent image, and Lorentz forces on the chargedtoner particles due to the electric field produced by the bias voltages.

In some embodiments, the development station 225 employs a two-componentdeveloper that includes toner particles and magnetic carrier particles.The exemplary development station 225 includes a magnetic core 227 tocause the magnetic carrier particles near toning shell 226 to form a“magnetic brush,” as known in the electrophotographic art. Magnetic core227 can be stationary or rotating, and can rotate with a speed anddirection the same as or different than the speed and direction oftoning shell 226. Magnetic core 227 can be cylindrical ornon-cylindrical, and can include a single magnet or a plurality ofmagnets or magnetic poles disposed around the circumference of magneticcore 227. Alternatively, magnetic core 227 can include an array ofsolenoids driven to provide a magnetic field of alternating direction.Magnetic core 227 preferably provides a magnetic field of varyingmagnitude and direction around the outer circumference of toning shell226. Development station 225 can also employ a mono-component developercomprising toner, either magnetic or non-magnetic, without separatemagnetic carrier particles.

Transfer subsystem 50 includes transfer backup member 113, andintermediate transfer member 112 for transferring the respective printimage from photoreceptor 206 of imaging member 111 through a firsttransfer nip 201 to surface 216 of intermediate transfer member 112, andthen to a receiver 42 which receives respective toned print images 38from each printing subsystem in superposition to form a composite imagethereon. The print image 38 is, for example, a separation of one color,such as cyan. Receiver 42 is transported by transport web 81. Transferto a receiver is effected by an electrical field provided to transferbackup member 113 by power source 240, which is controlled by LCU 99.Receiver 42 can be any object or surface onto which toner can betransferred from imaging member 111 by application of the electricfield. In this example, receiver 42 is shown prior to entry into asecond transfer nip 202, and receiver 42 a is shown subsequent totransfer of the print image 38 onto receiver 42 a.

In the illustrated embodiment, the toner image is transferred from thephotoreceptor 206 to the intermediate transfer member 112, and fromthere to the receiver 42. Registration of the separate toner images isachieved by registering the separate toner images on the receiver 42, asis done with the NexPress 2100. In some embodiments, a single transfermember is used to sequentially transfer toner images from each colorchannel to the receiver 42. In other embodiments, the separate tonerimages can be transferred in register directly from the photoreceptor206 in the respective printing subsystem 31, 32, 33, 34, 25 to thereceiver 42 without using a transfer member. Either transfer process issuitable when practicing this invention. An alternative method oftransferring toner images involves transferring the separate tonerimages, in register, to a transfer member and then transferring theregistered image to a receiver.

LCU 99 sends control signals to the charging subsystem 210, the exposuresubsystem 220, and the respective development station 225 of eachprinting subsystem 31, 32, 33, 34, 35 (FIG. 1), among other components.Each printing subsystem can also have its own respective controller (notshown) coupled to LCU 99.

Various finishing systems can be used to apply features such asprotection, glossing, or binding to the printed images. The finishingsystems can be implemented as integral components of the printer 100, orcan include one or more separate machines through which the printedimages are fed after they are printed.

FIG. 3 shows a conventional processing path that can be used to producea printed image 450 using a print engine 370. A pre-processing system305 is used to process a page description file 300 to provide image data350 that is in a form that is ready to be printed by the print engine370. In an exemplary configuration, the pre-processing system 305includes a digital front end (DFE) 310 and an image processing module330. The pre-processing system 305 can be a part of printer 100 (FIG.1), or may be a separate system which is remote from the printer 100.The DFE 310 and an image processing module 330 can each include one ormore suitably-programmed computer or logic devices adapted to performoperations appropriate to provide the image data 350.

The DFE 310 receives page description files 300 which define the pagesthat are to be printed. The page description files 300 can be in anyappropriate format (e.g., the well-known Postscript command file formator the PDF file format) that specifies the content of a page in terms oftext, graphics and image objects. The image objects are typicallyprovided by input devices such as scanners, digital cameras or computergenerated graphics systems. The page description file 300 can alsospecify invisible content such as specifications of texture, gloss orprotective coating patterns.

The DFE 310 rasterizes the page description file 300 into image bitmapsfor the print engine to print. The DFE 310 can include variousprocessors, such as a raster image processor (RIP) 315, a colortransform processor 320 and a compression processor 325. It can alsoinclude other processors not shown in FIG. 3, such as an imagepositioning processor or an image storage processor. In someembodiments, the DFE 310 enables a human operator to set up parameterssuch as layout, font, color, media type or post-finishing options.

The RIP 315 rasterizes the objects in the page description file 300 intoan image bitmap including an array of image pixels at an imageresolution that is appropriate for the print engine 370. For text orgraphics objects the RIP 315 will create the image bitmap based on theobject definitions. For image objects, the RIP 315 will resample theimage data to the desired image resolution.

The color transform processor 320 will transform the image data to thecolor space required by the print engine 370, providing colorseparations for each of the color channels (e.g., CMYK). For cases wherethe print engine 370 includes one or more additional colors (e.g., red,blue, green, gray or clear), the color transform processor 320 will alsoprovide color separations for each of the additional color channels. Theobjects defined in the page description file 300 can be in anyappropriate input color space such as sRGB, CIELAB, PCS LAB or CMYK. Insome cases, different objects may be defined using different colorspaces. The color transform processor 320 applies an appropriate colortransform to convert the objects to the device-dependent color space ofthe print engine 370. Methods for creating such color transforms arewell-known in the color management art, and any such method can be usedin accordance with the present invention. Typically, the colortransforms are defined using color management profiles that includemulti-dimensional look-up tables. Input color profiles are used todefine a relationship between the input color space and a profileconnection space (PCS) defined for a color management system (e.g., thewell-known ICC PCS associated with the ICC color management system).Output color profiles define a relationship between the PCS and thedevice-dependent output color space for the printer 100. The colortransform processor 320 transforms the image data using the colormanagement profiles. Typically, the output of the color transformprocessor 320 will be a set of color separations including an array ofpixels for each of the color channels of the print engine 370 stored inmemory buffers.

The processing applied in digital front end 310 can also include otheroperations not shown in FIG. 3. For example, in some configurations, theDFE 310 can apply the halo correction process described incommonly-assigned U.S. Pat. No. 9,147,232 (Kuo) entitled “Reducing haloartifacts in electrophotographic printing systems,” which isincorporated herein by reference.

The image data provided by the digital front end 310 is sent to theimage processing module 330 for further processing. In order to reducethe time needed to transmit the image data, a compressor processor 325is typically used to compress the image data using an appropriatecompression algorithm. In some cases, different compression algorithmscan be applied to different portions of the image data. For example, alossy compression algorithm (e.g., the well-known JPEG algorithm) can beapplied to portions of the image data including image objects, and alossless compression algorithm can be applied to portions of the imagedata including binary text and graphics objects. The compressed imagevalues are then transmitted over a data link to the image processingmodule 330, where they are decompressed using a decompression processor335 which applies corresponding decompression algorithms to thecompressed image data.

A halftone processor 340 is used to apply a halftoning process to theimage data. The halftone processor 340 can apply any appropriatehalftoning process known in the art. Within the context of the presentdisclosure, halftoning processes are applied to a continuous-tone imageto provide an image having a halftone dot structure appropriate forprinting using the printer module 435. The output of the halftoning canbe a binary image or a multi-level image. In an exemplary configuration,the halftone processor 340 applies the halftoning process described incommonly assigned U.S. Pat. No. 7,830,569 (Tai et al.), entitled“Multilevel halftone screen and sets thereof,” which is incorporatedherein by reference. For this halftoning process, a three-dimensionalhalftone screen is provided that includes a plurality of planes, eachcorresponding to one or more intensity levels of the input image data.Each plane defines a pattern of output exposure intensity valuescorresponding to the desired halftone pattern. The halftoned pixelvalues are multi-level values at the bit depth appropriate for the printengine 370.

The image enhancement processor 345 can apply a variety of imageprocessing operations. For example, an image enhancement processor 345can be used to apply various image enhancement operations. In someconfigurations, the image enhancement processor 345 can apply analgorithm that modifies the halftone process in edge regions of theimage (see U.S. Pat. No. 7,079,281, entitled “Edge enhancement processorand method with adjustable threshold setting” and U.S. Pat. No.7,079,287 entitled “Edge enhancement of gray level images” (both to Nget al.), and both of which are incorporated herein by reference).

The pre-processing system 305 provides the image data 350 to the printengine 370, where it is printed to provide the printed image 450. Thepre-processing system 305 can also provide various signals to the printengine 370 to control the timing at which the image data 350 is printedby the print engine 370. For example, the pre-processing system 305 cansignal the print engine 370 to start printing when a sufficient numberof lines of image data 350 have been processed and buffered to ensurethat the pre-processing system 305 will be capable of keeping up withthe rate at which the print engine 370 can print the image data 350.

A data interface 405 in the print engine 370 receives the data from thepre-processing system 305. The data interface 405 can use any type ofcommunication protocol known in the art, such as standard Ethernetnetwork connections. A printer module controller 430 controls theprinter module 435 in accordance with the received image data 350. In anexemplary configuration, the printer module 435 can be the printer 100of FIG. 1, which includes a plurality of individual electrophotographicprinting subsystems 31, 32, 33, 34, 35 for each of the color channels.For example, the printer module controller 430 can provide appropriatecontrol signals to activate light sources in the exposure subsystem 220(FIG. 2) to exposure the photoreceptor 206 with an exposure pattern. Insome configurations, the printer module controller 430 can apply variousimage enhancement operations to the image data. For example, analgorithm can be applied to compensate for various sources ofnon-uniformity in the printer 100 (e.g., streaks formed in the chargingsubsystem 210, the exposure subsystem 220, the development station 225or the fuser module 60. One such compensation algorithm is described incommonly-assigned U.S. Pat. No. 8,824,907 (Kuo et al.), entitled“Electrophotographic printing with column-dependent tonescaleadjustment,” which is incorporated herein by reference.

In the configuration of FIG. 3, the pre-processing system 305 is tightlycoupled to the print engine 370 in that it must supply image data 350 ina state which is matched to the printer resolution and the halftoningstate required for the printer module 435. As a result, when newversions of the print engine 370 having different printer resolutions orhalftone state requirements are developed, it has been necessary to alsoprovide an updated version of the pre-processing system 305 thatprovides image data 350 in an appropriate state. This has thedisadvantage that customers are required to upgrade both thepre-processing system 305 and the print engine 370 at the same time,both of which can have significant costs. The present inventionaddresses this problem by providing an improved print engine designwhich is compatible with a variety of different pre-processing systems.

Aspects of the present invention will now be described with reference toFIG. 4, which shows an improved print engine 400 that is adapted toproduce printed images 450 from image data 350 provided by a pluralityof different pre-processing systems 305 that are configured to supplyimage data 350 having different image resolutions and halftoning states.In an exemplary configuration, the pre-processing systems 305 aresimilar to that discussed with respect to FIG. 3, and includes a digitalfront end 310 and an image processing module 330. Details of theprocessing provided by the digital front end 310 and an image processingmodule 330 are not included in FIG. 4 for clarity, but will be analogousto the processing operations that were discussed with respect to FIG. 3.In this case, in addition to supplying image data 350, thepre-processing system 305 also supplies appropriate metadata 360 thatprovides an indication of the state of the image data 350. Inparticular, the metadata 360 provides an indication of the imageresolution and the halftoning state of the image data 350.

In an exemplary configuration, the metadata 360 includes an imageresolution parameter that provides an indication of an image resolutionof the image data 350 provided by the pre-processing system 305 and ahalftone state parameter that provides an indication of a halftoningstate of the image data provided by the pre-processing system 305.

The image resolution parameter (R) can take any appropriate form thatconveys information about the image resolution of the image data 350. Insome embodiments, the image resolution parameter can be an integerspecifying the spatial resolution in appropriate units such as dots/inch(dpi) (e.g., R=600 for 600 dpi and R=1200 for 1200 dpi). In otherembodiments, the image resolution parameter can be an index to anenumerated list of allowable spatial resolutions (e.g., R=0 for 600 dpiand R=1 for 1200 dpi).

The halftone state parameter (H) can also take any appropriate form. Insome embodiments, the halftone state parameter can be a Boolean variableindicating whether or not a halftoning process was applied in thepre-processing system 305 such that the image data 350 is in a halftonedstate (e.g., H=FALSE indicates that a halftoning process was not appliedso that the image data 350 is in a continuous tone state, and H=TRUEindicates that a halftoning process was applied so that the image data350 is in a halftoned state.) In other embodiments, when thepre-processing system 305 applied a halftoning process, the halftonestate parameter can also convey additional information about the type ofhalftoning process that was applied. For example, the halftone stateparameter can be an integer variable, where H=0 indicates that nohalftoning process was applied, and other integer values represent anindex to an enumerated list of available halftoning states (e.g.,different screen frequency/angle/dot shape combinations).

The metadata 360 can also specify other relevant pieces of information.For example, for the case where the image data 350 is in a continuoustone state such that a halftone processor 425 in the print engine 400will be required to apply a halftoning operation, the metadata 360 canalso include one or more halftoning parameters that are used by thehalftone processor 425 to control the halftoning operation. In someembodiments, the halftoning parameters can include a screen angleparameter, a screen frequency parameter, or a screen type parameter. Inother embodiments, the halftoning parameters can include a halftoneconfiguration index that is used to select one of a predefined set ofhalftone algorithm configurations.

The print engine 400 receives the image data 350 and the metadata 360using an appropriate data interface 405 (e.g., an Ethernet interface).The print engine includes a metadata interpreter 410 that analyzes themetadata 360 to provide appropriate control signals 415 that are used tocontrol a resolution modification processor 420 and a halftone processor425, which are used to process the image data 350 to provide processedimage data 428, which is in an appropriate state to be printed by theprinter module 435. Printer module controller 430 then controls theprinter module 435 to print the processed image data 428 to produce theprinted image 450 in an analogous manner to that which was discussedrelative to FIG. 3.

FIG. 5 shows additional details of the resolution modification processor420 and the halftone processor 425 of FIG. 4 according to an exemplaryconfiguration. In this example, the control signals 415 provided by themetadata interpreter 410 (FIG. 4) in response to analyzing the metadata360 (FIG. 4) include a resolution modification flag 416, a resize factor417, a halftoning flag 418 and halftoning parameters 419.

The resolution modification flag 416 provides an indication of whether aresolution modification must be performed. In an exemplary configurationthe resolution modification flag 416 is a Boolean variable that would beset to FALSE if no resolution modification is required (i.e., if theimage resolution of the image data 350 matches the printer resolution ofthe printer module 435), and would be set to TRUE of a resolutionmodification is required. The halftoning flag 418 provides an indicationof whether a halftoning operation is required. In an exemplaryconfiguration the halftone flag 418 is a Boolean variable that would beset to FALSE if no halftoning operation is required (i.e., if the imagedata 350 is in a halftoning state that is appropriate for the printermodule 435), and would be set to TRUE if a halftoning operation must beapplied to the image data 350 before it is ready to be printed.

The resolution modification processor 420 applies modify resolution test421 to determine whether a resolution modification should be performedresponsive to the resolution modification flag 416. If a resolutionmodification is required, a resolution modification operation 422 isperformed. In some configurations, the metadata interpreter 410 (FIG. 4)provides a resize factor 417 that specifies the amount of resizing thatmust be provided to adjust the resolution of the image data 350 to theresolution required by the printer module 435 (FIG. 4). In someconfigurations, the resize factor 417 is a variable specifying the ratiobetween the printer resolution and the image resolution. For example, ifthe image data 350 is at 600 dpi and the printer module 435 prints at1200 dpi, the resize factor 417 would specify that a 2× resolutionmodification is required. In various configurations the resize factor417 could be greater than 1.0 if the printer module 435 has a higherresolution than the image data 350, or it could be less than 1.0 if theprinter module 435 has a lower resolution than the image data 350.

In an exemplary configuration, if the image resolution of the image data350 supplied by the pre-processing system 305 is an integer fraction ofthe printer resolution of the printer module 435 so that the resizefactor 417 is a positive integer, the resolution modification operation422 performs the resolution modification by performing a pixelreplication process. For example, each 600 dpi image pixel in the imagedata 350 would be replaced with a 2×2 array of 1200 dpi image pixels,each having the same pixel value. In other configurations, anappropriate interpolation process can be used by the resolutionmodification operation 422 (e.g., nearest neighbor interpolation,bi-linear interpolation or bi-cubic interpolation). The use of aninterpolation algorithm is particularly useful of the resize factor isnot an integer.

For cases where the resize factor is less than 1.0, the resolutionmodification operation 422 can perform appropriate averaging operationsto avoid aliasing artifacts. For example, if the resize factor 417 is0.5, then 2×2 blocks of image pixels in the image data 350 can beaveraged together to provide the new resolution. In otherconfigurations, the resolution modification operation 422 can apply alow-pass filter operation followed by a resampling operation.

The halftone processor 425 applies halftone image test 426 to determinewhether a halftoning operation should be performed responsive to thehalftoning flag 418. If a halftoning operation is required (e.g., if theimage data 350 is in a continuous-tone state), a halftoning operation427 is performed. In some configurations, the metadata interpreter 410(FIG. 4) provides one or more halftoning parameters 419 that are used tocontrol the halftoning operation. As discussed earlier, the halftoningparameters 419 can include a screen angle parameter, a screen frequencyparameter, or a screen type parameter. In other embodiments, thehalftoning parameters 419 can include a halftone configuration indexthat is used to select one of a predefined set of halftone algorithmconfigurations.

The halftoning operation applied by the halftone processor 425 can useany appropriate halftoning algorithm known in the art. In someembodiments, any of the halftoning algorithms described incommonly-assigned U.S. Pat. No. 7,218,420 (Tai et al.), entitled “Graylevel halftone processing,” commonly-assigned U.S. Pat. No. 7,626,730(Tai et al.), entitled “Method of making a multilevel halftone screen,”and commonly-assigned U.S. Pat. No. 7,830,569 (Tai et al.), entitled,“Multilevel halftone screen and sets thereof,” each of which areincorporated herein by reference, can be used. Such halftoningalgorithms typically involve defining look-up tables defining thehalftone dot shape as a function of position for a tile of pixels.Different look-up tables can be specified to produce different halftonedot patterns. For example, different look-up tables can be specified fordifferent screen angles, screen frequencies and dot shapes. In thiscase, the halftoning parameters 419 can include a halftone configurationindex that selects which look-up table should be used to halftone theimage data 350.

Consider the case where the printer module 435 prints halftoned imagedata at 1200 dpi, but where different pre-processing systems 305 andconfigurations can be used to supply image data 350 at either 600 dpi or1200 dpi, and in either a halftoned state or a continuous tone state. Inthis case, there will be four different combinations of the imageresolution parameters and the halftone state parameters that the printengine must deal with.

-   1. The image resolution parameter indicates that image data 350 is    600 dpi, and the halftone state parameter indicates that the image    data 350 is in a halftoned state. In this case, the print engine 400    would print the image data 350 in a mode that emulates a 600 dpi    printer. The resolution modification processor 420 would be used to    modify the image resolution to provide the 1200 dpi data required by    the printer module 435. In an exemplary embodiment, each 600 dpi    image pixel is replicated to provide a 2×2 array of 1200 dpi image    pixels. Since the image data is already in a halftoned state, the    halftone operation 427 would be bypassed.-   2. The image resolution parameter indicates that the image data 350    is 600 dpi, and the halftone state parameter indicates that the    image data 350 is in a continuous-tone state. In this case, the    resolution modification processor 420 would be used to modify the    image resolution to provide the 1200 dpi data appropriate for the    printer module 435, and the halftone processor 425 would apply a    halftoning operation 427 to the 1200 dpi image data in accordance    with the halftoning parameters 419.-   3. The image resolution parameter indicates that the image data 350    is 1200 dpi, and the halftone state parameter indicates that the    image data 350 is in a halftoned state. In this case, the image data    350 is already in a state that is ready for printing by the printer    module 435, therefore the resolution modification operation 422 and    the halftoning operation 427 would both be bypassed.-   4. The image resolution parameter indicates that image data 350 is    1200 dpi, and the halftone state parameter indicates that the image    data 350 is in a continuous-tone state. In this case, since the    image data is already at 1200 dpi so that the resolution    modification operation 422 would be bypassed, and the halftone    processor 425 would apply a halftoning operation 427 to the 1200 dpi    image data in accordance with the halftoning parameters 419.

In a preferred configuration, the halftone processor 425 uses acomputational halftone process to compute halftoned pixel values using adefined set of calculations. The calculations can be performed in aprocessor (e.g., a field-programmable gate array) located in the printengine 370. For example, the halftone processor 425 can determinehalftoned pixel values E(x,y) for each (x,y) pixel position in the imagedata 350 using the process outlined in FIG. 6. In summary, the processis used to determine halftoned pixels 545 (E(x,y)) using the followingrelationship:

$\begin{matrix}{{E( {x,y} )} = {\frac{1}{N \times N}{\sum\limits_{i = 1}^{N}{\sum\limits_{j = 1}^{N}{H( {u_{i},v_{j}} )}}}}} & (1)\end{matrix}$where the halftoned pixel values E(x,y) define the pattern of exposurevalues to be provided to the photoreceptor 206 (FIG. 2) by the exposuresubsystem 220 (FIG. 2). The halftoned pixel values E(x,y) are determinedby averaging halftoned pixel values computed using a halftone dotfunction 530 (H(u,v)) for an array of high-resolution dot coordinates520 (u_(i),v_(j)) determined by performing a coordinate transformationto an N×N array of high-resolution printer coordinates 510(x_(i),y_(j)). N is a positive integer greater than one. Preferably,N≥3. In an exemplary configuration, N=5. In a preferred embodiment, thehalftone dot function 530 defines a center-growing dot pattern. However,other types of dot patterns can also be used.

One skilled in the art will recognize that Eq. (1) has the effect ofcomputing high-resolution halftoned pixel values at a higher resolutionthan the native printer resolution, and then averaging them down todetermine the final halftoned pixel values E(x,y) at the printerresolution. In this case, high-resolution halftoned pixel values 535(H(u_(i), v_(j))) are determined for an N×N array of sub-pixels for eachinput pixel, such that the high-resolution halftoned exposure values aredetermined at a resolution that is NX greater than the printerresolution.

For each input pixel 500 (C(x,y)) of the image data 350, a definehigh-resolution printer coordinates step 505 is used to define an arrayof high-resolution printer coordinates 510 (x_(i),y_(j)) by sub-samplingthe printer coordinates at a higher resolution than the printerresolution in a neighborhood around the (x,y) pixel position.

In an exemplary configuration, the array of high-resolution printercoordinates 510 can be determined using the following relationship:x _(i) =x+i/Ny _(j) =y+j/N  (2)where (x,y) are the pixel coordinates of the input pixel 500 in theprinter coordinate system, and where i and j are array indices whichrange from 0 to N−1. One skilled in the art will recognize that this hasthe effect of defining an N×N array of high-resolution printercoordinates 510 in a neighborhood of the (x,y) pixel coordinate bydefining a set of intermediate positions between the pixel coordinatesof the image data 350. The resulting high-resolution printer coordinates510 are sampled at a higher spatial resolution than the printerresolution associated with the image data 350.

A determine high-resolution dot coordinates step 515 is used todetermine a corresponding array of high-resolution dot coordinates 520(u_(i),v_(j)) by applying an appropriate coordinate transformation tothe high-resolution printer coordinates 510 (x_(i),y_(j)). Thecoordinate transformation which will be a function of the screen angleand screen frequency of the halftone pattern. In an exemplaryembodiment, the coordinate transformation is performed using thefollowing equation:

$\begin{matrix}{\begin{bmatrix}u_{i} \\v_{j}\end{bmatrix} = {{\begin{bmatrix}{1/\alpha} & 0 \\0 & {1/\alpha}\end{bmatrix}\begin{bmatrix}{\cos(\theta)} & {\sin(\theta)} \\{- {\sin(\theta)}} & {\cos(\theta)}\end{bmatrix}}\begin{bmatrix}x_{i} \\y_{j}\end{bmatrix}}} & (3)\end{matrix}$where θ=screen angle, andα=f _(p) /f _(h)  (4)where f_(h) is the halftone screen frequency (lines/inch), and f_(p) isthe printer resolution (dots/inch). The printer resolution can also bereferred to as pixel frequency of the printer. The first matrix in Eq.(3) scales the printer coordinates in accordance with the ratio α, andthe second matrix in Eq. (3) rotates the printer coordinates inaccordance with the screen angle θ.

A determine high-resolution halftoned pixel values step 525 is used todetermine a set of high-resolution halftoned pixel values 535(H(u_(i),v_(j))) using halftone dot function 530 corresponding to thearray of high-resolution dot coordinates 520 (u_(i),v_(j)). The halftonedot function 530 defines the desired halftone dot value (e.g., theexposure level) as a function of the input image code value C(x,y) andthe relative position within the halftone dot as specified by thehigh-resolution dot coordinates 520. In a preferred embodiment, thehalftone dot function 530 is defined using following relationship:H(u,v)=T _(E)(h(u,v))  (5)where h(u,v) is a halftone shape function specifying the halftone dotshape as a function of the dot coordinates (u,v), and T_(E)(h) is anedge shape function. The edge shape function will generally vary as afunction of code value C.

In an exemplary configuration, the halftone shape function h(u,v) can becomputed using the following relationship:h(u,v)=(u′ ^(p) +v′ ^(p))^(1/p)  (6)where p is a dot shape parameter (where p=1 produces diamond shaped dotsand p=2 produces circular dots), and where u′ and v′ represent therelative position within the halftone cell, which can be calculated by:u′=|(u−Int(u))−0.5|v′=|(v−Int(v))−0.5|  (7)where Int(•) is a function that returns the integer portion of a number.Eq. (7) takes advantage of the fact that the dot function will besymmetric around the center of the halftone cell having coordinates(0.5, 0.5).

In some configurations, the value of the dot shape parameter p can be afunction of the code value C so that the dot shape can be variedthroughout the tonescale. FIG. 7 shows an example of a dot shapefunction 600 where the dot shape transitions from circular dots for lowcode values to diamond shaped dots in the mid-tones, and then back tocircular dots for high code values. (The dot shape function 600 in FIG.7 is specified in terms of a normalized code value, which can becomputed by dividing an integer code value by the maximum code value.)

The edge shape function T_(E)(h) maps the halftone dot shape values tocorresponding halftoned pixel values, and can take any appropriate form.At one extreme, the edge shape function can take the form of a hardthreshold. In other cases, the edge shape function can be used toimplement a soft threshold such that the resulting halftone dots willhave a soft edge. In an exemplary configuration, the edge shape functionT(h) can take the following form:

$\begin{matrix}{{T_{E}(h)} = \{ \begin{matrix}{{1;}\ } & {h < T_{L}} \\{{1 - \frac{h - T_{L}}{T_{H} - T_{L}}}\ } & {T_{L} \leq h \leq T_{H}} \\{{0;}\ } & {h > T_{H}}\end{matrix} } & (8)\end{matrix}$where h is the halftone shape value, and T_(L) and T_(H) are low andhigh threshold values, respectfully. In an exemplary configuration, thelow and high threshold values can be computed by:T _(L) =T(C)−ΔT(C)T _(H) =T(C)+ΔT(C)  (9)where T(C) is a threshold value function which indicates where thehalftone function h(u,v) should be thresholded to produce the desiredtone scale, and ΔT(C) is an edge softness function which controls thesoftness of the edges of the halftone dots. Both T(C) and ΔT(C) arefunctions of the image code value (C).

The edge shape function T_(E)(h) given in Eq. (8) includes a lineartransition from the low threshold value T_(L) to the high thresholdvalue T_(H). In alternate configurations, a non-linear function can beused such as a sigmoid function.

The threshold value function T(C) can be used to account variousattributes of the printer 100 (FIG. 1) to produce a desired tonescalerelationship (e.g., density as a function of input code value C). Forexample, the threshold value function T(C) can be specified tocompensate for dot gain characteristics of the print engine 400 (FIG.4). In some embodiments, the threshold value function is determinedusing a calibration process that is performed during a system set-upprocess. FIG. 8 shows an example of a typical threshold value function605 that produces a specified aim tonescale.

FIG. 9 shows examples of edge softness functions ΔT(C) that can be usedto provide halftone dots having different levels of edge softness. Edgesoftness function 610 produces relatively hard edges on the halftonedots, and edge softness function 615 produces relatively soft edges onthe halftone dots.

Returning to a discussion of FIG. 6, once the array of high-resolutionhalftoned pixel values 535 are determined, an average high-resolutionhalftoned pixel values step 540 is used to average the high-resolutionhalftoned pixel values 535 to determine the halftoned pixel value E(x,y)for the halftone pixel 545. The averaging process is reflected by thesummation and normalization shown in Eq. (1).

A more pixels test 550 is then used to determine if all of the pixels inthe image data 350 have been processed. If more pixels remain to beprocessed, the computational halftone process repeats the stepsdescribed above the next input pixel 500.

FIG. 10 shows halftoned images 620, 625 determined using the method ofFIG. 6 and the computations specified in Eqs. (1)-(9). The dot shapefunction 600 of FIG. 7 was used for both of the halftoned images 620,625. Consequently, it can be seen that the circular dots are formed inthe light and dark ends of the tonescale, and that diamond-shaped dotsare formed in the mid-tone portion of the tonescale. The halftoned image620 was formed using the edge softness function 610 of FIG. 8 so thatthe resulting halftone dots have relatively hard edges, and thehalftoned image 625 was formed using the edge softness function 615 ofFIG. 8 so that the resulting halftone dots have relatively soft edges.

It will be obvious to one skilled in the art that various methods can beused to improve the computational efficiency the computations describedabove with reference to FIG. 6. For example, various non-linearfunctions, such as the halftone shape function given in Eq. (6), can beimplemented using look-up tables to avoid the need to performcalculations such as power functions which can be computationallyexpensive.

The use of the computational halftoning process described with referenceto FIG. 6 provides a number of advantages. Most prior art halftoningprocesses are typically limited to processing 8-bit pixel values. Forexample, many halftoning operations utilize a set of look-up tablesdefining the halftone dot shape as a function of position for a tile ofpixels. A different look-up table is often provided for each of 256different input pixel levels. Since the computational halftoning processdescribed herein uses a set of equations rather than a set of predefinedhalftone look-up tables to determine the halftoned pixel values, it caneasily be used to process input pixels having any bit-depth. This beparticularly important where calibration functions are applied to theimage data 350 before applying the halftoning process. If the output ofthe calibration process is limited to an 8-bit value, it has been foundthat the resulting image can be susceptible to quantization artifacts insome cases. As a result, it is preferable that the calibrated image datahave a bit-depth of at least 10 bits.

Another advantage of the computational halftoning process is that it canproduce halftoned images having an arbitrary screen angle and screenfrequency without producing aliasing artifacts. Halftoning operationsthat utilize look-up tables defining the halftone dot shape for a tileof pixels can support only certain screen angle/screen frequencycombinations due to the tiling constraint. Various artifacts have beenobserved in printers 100 where image regions that are intended to haveuniform tone levels exhibit non-uniformities.

These non-uniformities can result from a variety of different sourcessuch as variations in the light output of the individual light sourcesin the printhead of the exposure subsystem 220 (FIG. 2), ornon-uniformities in other system components such as the photoreceptor206 (FIG. 2), the charging subsystem 210 (FIG. 2), the developmentstation 225 (FIG. 2) or the fuser module 60 (FIG. 1). The magnitudes ofthe non-uniformity artifacts can generally be a function of thecross-track and in-track positions within the printed image, as well asthe printed tone-level. Such non-uniformities can result in artifactssuch as image streaks, where the amplitude of the streaks can depend onthe tone-level of the image area. There can also be random components tothe non-uniformity artifacts which can result in noise or “graininess”in the image.

Commonly-assigned U.S. Pat. No. 10,036,975 to Kuo et al., entitled“Determining a pulse timing function for a linear printhead,”commonly-assigned U.S. Pat. No. 10,126,696 to Kuo et al., entitled“Adaptive printhead calibration process,” and commonly-assigned U.S.Pat. No. 10,192,150 to Kuo et al., entitled “Print engine withprint-mode-dependent pulse timing functions,” each of which isincorporated herein by reference, describe aspects of a system andprocesses that can be used to reduce non-uniformity artifacts associatedwith printheads including a linear array of light sources. However, evenwhen such processes are employed there can be residual non-uniformityartifacts that can be visible and objectionable in some applications.The present invention provides a method for further reducing tone-levelnon-uniformities in a digital printing system. In some embodiments, themethod can be combined with other correction processes such as thosedescribed in the above-referenced patents.

Commonly-assigned U.S. Pat. No. 8,482,802 to Tai et al., entitled“Screened hardcopy reproduction apparatus with compensation,” which isincorporated herein by reference describes a method for compensating fornon-uniformity artifacts in an image. The method includes determining acorrection value using a 2-D look-up table addressed by cross-trackposition and output pixel level. It has the disadvantage that the 2-Dlook-up table requires a large amount of memory for storage.Additionally, the corrections were applied to 8-bit/pixel image data,which was found to produce visible contouring artifacts in someapplications.

FIG. 11 shows an improved processing path including a print engine thatis adapted to produce printed images incorporating tone-levelcorrections to correct for tone-level non-uniformities in accordancewith an exemplary embodiment, where the characteristics of thetone-level non-uniformities vary with tone-level and cross-trackposition, and optionally with in-track position. The improved processingpath is analogous to the processing path of FIG. 4 except that theresolution modification processor 420 has been replaced by aresolution/tone-level processor 800, which corrects the tone-levelnon-uniformities responsive to the tone-level correction function 805 inaddition to performing any resolution modifications specified by thecontrol signals 415.

FIG. 12 shows additional details for the resolution/tone-level processor800 and the halftone processor 425 of FIG. 11. This process is similarto that of FIG. 5 except for the addition of a tone-level correctionoperation 810. As discussed earlier, the resolution modificationoperation 422 involves resampling the image data 350 in accordance witha resize factor. The tone-level correction operation 810 involvesmodifying the tone-level of the image data as a function of thetone-level and the cross-track position, and optionally as a function ofthe in-track position. This operation is preferably applied tohigh-bit-depth image data in order to avoid the introduction oftone-level contouring artifacts. Preferably, the high-bit-depth imagedata is at least 10-bits/pixel image data, and more preferably it is atleast 12-bits/pixel image data. In an exemplary embodiment, the imagedata 350 comes into the print engine 400 (FIG. 11) in an 8-bit/pixelformat, but is converted to a 12-bit/pixel format before it is passedinto the resolution/tone-level processor 800. The bit-depth conversionis typically performed with a look-up table, which may be linear ornon-linear. The halftoning operation 427 is preferably applied to thesame high-bit-depth image data in order to preserve the tone-levelcorrections without introducing tone-level contouring. The exemplarycomputational halftoning process described earlier with respect to FIG.6 is well-suited to the present application.

FIG. 13 shows a flowchart of an exemplary method for determining atone-level correction function 805 that characterizes corrections thatcan be used to correct for tone-level non-uniformities that vary withtone-level and position within the image (i.e., cross-track position andoptionally in-track position). The method includes providing digitalimage data for a test target 900. The test target 900 preferablyincludes a plurality of alignment marks 570 positioned at predefinedcross-track positions as illustrated in the exemplary arrangement shownin FIG. 14. The alignment marks 570 are preferably distributed along thelength of the printhead 475 which spans the test target 900 in thecross-track direction 590. The test target 900 also include uniform testpatches 575 having code values that correspond to a set of differenttone-levels. The uniform test patches 575 are useful for characterizingnon-uniformities in the reproduced tone-level that vary with tone-leveland position. In the exemplary arrangement of FIG. 14, the test target900 includes a set of five uniform test patches 575 that span the testtarget 900 in the cross-track direction 590 for each of a plurality ofdifferent color channels. While a set of five uniform test patches 575is shown for illustration purposes, different numbers of test patchescan be used in practice. In a preferred embodiment, eight test patchesare used in the set of uniform test patches 575. In the illustratedexample, the test target includes first color channel image content 580for a first color channel printed by a first printing subsystem 31 (FIG.1), second color channel image content 581 for a second color channelprinted by a second printing subsystem 32 (FIG. 1), third color channelimage content 582 for a third color channel printed by a third printingsubsystem 32 (FIG. 1), and fourth color channel image content 583 for afourth color channel printed by a fourth printing subsystem 34 (FIG. 1).The image content for each color channel is provided in different imageregions distributed in the in-track direction 595. The different colorchannels can be, for example, black, cyan, magenta and yellow. However,one skilled in the art will recognize that the color channels can useother colorants as well. In the illustrated configuration, each ofdifferent image regions includes a corresponding set of alignment marks570 in addition to the set of uniform test patches 575. In otherembodiments, rather than using a single test target 900 includinguniform test patches 575 for all of the color channels, they can beincluded in a plurality of test targets 900 (e.g., one for each colorchannel). For cases where it is desirable to characterize tone-levelnon-uniformities that vary as a function of in-track position, each ofthe uniform test patches 575 can fill a single page in order to be ableto assess the tone-level non-uniformities over the entire area of theimage.

The alignment marks 570 are useful to determine the cross-track positionwithin the test target. Preferably, at least ten alignment marks 570 areprovided across the cross-track width of the test target 900 (i.e.,along the length of the printhead 475) to enable the accuratedetermination of cross-track position. As described incommonly-assigned, co-pending U.S. patent application Ser. No.16/417,731, entitled: “Correcting cross-track errors in a linearprinthead”, by Kuo et al., which is incorporated herein by reference,the alignment marks can also be used to characterize and correct forcross-track position errors.

Returning to a discussion of FIG. 13 a print test target step 905 isused to print the test target 900 to produce a printed test target 910.In a preferred embodiment, the printed test target 910 is formed on apiece of receiver 42 (FIG. 2) such as a sheet of paper. In other cases,the printed test target 910 can be an image transferred directly ontothe transport web 81 rather than onto a sheet of receiver 42. In otherembodiments, the printed test target 910 can correspond to anintermediate image formed on the surface of the imaging member 111(i.e., the photoreceptor 206) or the surface 216 of an intermediatetransfer member 112 (see FIG. 2).

A capture image step 915 is next used to capture a digital image of theprinted test target 910 using a digital image capture system to providea captured image 920. In an exemplary embodiment, the digital imagecapture system is a flatbed scanner external to the printer 100 which isused to scan the printed test target 910 formed on a receiver 42 afterit has been completely printed and fused. In other embodiments, adigital image capture system (e.g., a digital scanner system or adigital camera system) which is integrated into the printer 100 can beused to capture an image of the printed test target 910 on the receiver42 while the receiver 42 is traveling through the printer 100 (e.g.,while it is being carried on the transport web 81), or before it hasbeen transferred to the receiver 42 (e.g., on surface of the imagingmember 111 or the intermediate transfer member 112).

Next, an analyze captured image step 925 is used to automaticallyanalyze the captured image 920 to determine measured tone levels 930.The measured tone-levels 930 characterize the tone-level of each of theuniform test patches 575 (FIG. 14) as a function of cross-trackposition, and optionally as a function of in-track position. For thecase where the measured tone-levels are only determined as a function ofthe cross-track position, it will generally be desirable to average thetone-levels for a given cross-track position across a range of in-trackpositions within the uniform test patches 575. In some embodiments, theaveraging process can be performed by applying a one-dimensionallow-pass filter in the in-track direction and then taking a tracethrough the center of each uniform test patch 575. Since the printedtest target 910 will generally be a halftoned image, the filteringprocess can also apply a two-dimensional de-screening filter to averageout the halftoning pattern. This is particularly useful for the case ofimages halftoned with an AM screening algorithm. In some embodiments,the one-dimensional low-pass filter can be combined with thede-screening filter to form a single two-dimensional filter. Preferably,any skew in the captured image 920 can be characterized (e.g., bydetecting the boundaries of the uniform test patches 575) and accountedfor in the image analysis process. For example, the captured image 920can be rotated to remove the skew.

FIG. 15 shows a set of exemplary measured tone-level characteristics930. Each of the curves in FIG. 15 corresponds to a different uniformtest patch 575. The y-axis in this graph is given in terms of scannercode values, which will be related to the overall tone-level. The imagenon-uniformities (e.g., streak artifacts) show up as localizeddeviations of the individual curves.

Returning to a discussion of FIG. 13, a determine tone-level errors step940 is next used to determine tone level error characteristics 945 bycomparing the measured tone-level characteristics 930 with correspondingnominal tone-level characteristics 935. In an exemplary embodiment, thenominal tone-level characteristics 935 correspond to the aim tone-levelfor each of the uniform test patches 575 in the test target 900 asmeasured by the image capture system. In this case, the tone-level errorcharacteristics 945 will include any overall tone reproduction errors inaddition to characterizing the tone-level non-uniformities. However, inpractice the measured tone-level characteristics 930 will includenon-uniformities originating from sources such as scannernon-uniformities or paper deformations. It can be difficult to separatethese non-uniformities from the tone reproduction errors. Therefore, ina preferred embodiment, the nominal tone-level characteristics 935 aredetermined by smoothing the measured tone-level characteristics 930 toprovide a slowly-varying curve that characterizes any scanner or papernon-uniformities. In this case, the measured tone-level errorcharacteristics will exclude global tone reproduction errors and scannernon-uniformities and will only provide a representation of the localizedtone-level non-uniformities. Any global tone reproduction errors can beaccounted for in a separate tone scale calibration step. In an exemplaryembodiment, the measured tone-level characteristics 930 are smoothed byfitting a spline function to the data from each patch. The stiffness ofthe spline can be adjusted to fit the slowly-varying scannernon-uniformities while removing the localized tone-levelnon-uniformities that correspond to the desired tone-level errorcharacteristics 945.

In an exemplary embodiment, the tone-level error characteristics 945 arerepresented by a tone-level error function Δ_(s)(x,v):Δ_(s)(x,v)=I(x,v)−I ₀(x,v)  (10)where x is the cross-track position, I(x,v) is a representation of themeasured tone-level characteristics 930 as a function of cross-trackposition and tone level, and I₀(x,v) is a representation of thereference tone-level characteristics 935 as a function of cross-trackposition and tone level. Note that in an exemplary embodiment I(x,v) andI₀(x,v) are represented in terms of scanner code values such thattone-level error function Δ_(s)(x,v) will be represented in terms ofscanner code value differences. In other embodiments, a calibrationprocess can be used to relate the scanner code values to correspondinginput code values (i.e., the tone-level v) such that I(x,v) and I₀(x,v)are represented in terms of input code values and the tone-level errorfunction is represented in terms of corresponding input code valuedifferences.

FIG. 16 shows a set of exemplary tone-level error characteristics 945corresponding to the measured tone-level characteristics 930 of FIG. 15,where each of the individual graphs corresponds to a different one ofthe uniform test patches 575 having the indicated input code value v. Itcan be seen that the slowly-varying tone-level characteristics in themeasured tone-level characteristics 930 of FIG. 15, which generally comefrom sources such as scanner non-uniformities, have been removed toleave only the localized tone-level non-uniformities corresponding toartifacts such as image streak. In this example, the tone-level errorcharacteristics 945 in the graphs (i.e., the y-axis) is represented interms of scanner code value differences. As will be discussed later,these can be related to input code value differences by applying ascanner calibration function.

A determine tone-level correction function step 950 (FIG. 13) is used todetermine a tone-level correction function 805 that can be used tocompensate for the tone-level error characteristics 945. In an exemplaryembodiment, the tone-level correction function 805 is determined byinverting the determined tone-level error characteristics 945 andaccounting for the slope of a scanner calibration function which relatesthe scanner code values to corresponding input code values (i.e.,tone-level v):

$\begin{matrix}{{\Delta_{c}( {x,v} )} = {{- {\Delta_{s}( {x,v} )}} \cdot \frac{dv}{ds}}} & (11)\end{matrix}$where Δ_(c)(x,v) is the tone-level correction function 805, and dv/ds isthe slope of a scanner calibration function which relates the scannercode values to corresponding input code values evaluated at the measuredtone level.

In some embodiments, the scanner calibration function can be determinedusing a separate calibration process wherein a set of test patcheshaving known input code values are printed and measured. In an exemplaryembodiment the scanner calibration function can be determined directlyfrom the measured tone-level characteristics 930 for the test target900. For example, each uniform test patch 575 in the test target 900 hasa known input code value (i.e., tone-level v). The measured tone-levelcharacteristics 930 for each uniform test patch 575 can be analyzed todetermine a corresponding average scanner code value. (In someembodiments, the average scanner code value can be determined from thesmoothed nominal tone-level characteristics 935, and/or from only acentral portion of the uniform test patch to remove any scannerartifacts which are typically larger near the edges of the image.) Thesevalues will then define a set of data points on the scanner calibrationfunction. An interpolation process can then be used to fill in acomplete scanner calibration function.

FIG. 17 is a graph of an exemplary scanner calibration function 955which relates scanner code values to corresponding input code values.The eight data points for the scanner calibration function 955 weredetermined from the measured tone-level characteristics 930 of FIG. 15.A function is preferably fit to the measured data points to determine acontinuous function having an x-axis which spans the full range of inputcode values. In this example, a linear function was fit to the measureddata points. The slope of the linear calibration function isdv/ds=−20.1.

FIG. 18 shows data for an exemplary tone-level correction function 805determined from the tone-level error characteristics 945 of FIG. 16 andthe scanner calibration function 955 of FIG. 17 using Eq. (11). The datacorresponds to slices through the full tone-level correction function805 at the tone-levels of the uniform test patches 575. In someembodiments, a full tone-level correction function 805 is determined byinterpolating between the tone-levels of the uniform test patches 575.Preferably the interpolation process (e.g., a spline-fitting process) isconstrained such that the tone-level correction values are zero for theminimum and maximum tone-levels (e.g., v=0 and 4095).

The tone-level correction function 805 can be used to determinecorrected image data:v _(c)(x,y)=v(x,y)+Δ_(c)(x,v(x,y))  (12)where v(x,y) is the tone-level for a given input image pixel in theinput digital image at a particular cross-track position (x) andin-track position (y) within the image, and v_(c)(x,y) is the tone-levelfor the corresponding corrected image pixel. The term Δ_(c)(x,v(x,y))can be referred to as the tone-level correction for the image pixel atthe (x, y) position.

The tone-level correction function 805 can be stored in memory for usein correcting digital images in a variety of different formats. Forexample, the tone-level correction function 805 can be stored as atwo-dimensional look-up table which is addressed by the cross-trackposition (x) and the pixel value (v). In some embodiments, thetwo-dimensional look-up table can store tone-level corrections for everypossible cross-track position and pixel value. This can require arelatively large amount of memory given that the number of cross-trackpositions can be as large as 17,280 or more, and the number oftone-levels can be as large 4096 (corresponding to 12-bit image data) ormore. In some embodiments, the two-dimensional look-up table can storetone-level corrections for a sub-sampled set of cross-track positionsand pixel values in order to reduce the required amount of memory. Inthis case, a two-dimensional interpolation algorithm can be used todetermine tone-level corrections for cross-track positions and pixelvalues between the stored values.

In some embodiments, the tone-level correction function 805 can berepresented using an appropriate parametric function in order to reducethe amount of memory required to store the function. In a preferredembodiment, the tone-level correction function 805 (Δ_(c)(x,v)) isrepresented using a vector decomposition of the form:

$\begin{matrix}{{\Delta_{c}( {x,v} )} \cong {\sum\limits_{f = 1}^{n}{{i_{f}(x)}o\;{k_{f}(v)}}}} & (13)\end{matrix}$where i_(f)(x) and k_(s)(v) are one-dimensional feature vectors takenalong the cross-track and pixel-value axes of the tone-level correctionfunction 805, f is a feature vector index, n is the total number offeature vectors, and “∘” is the outer vector product operator. In anexemplary embodiment, data for the tone-level correction function 805such as that shown in FIG. 18 is stored in a two-dimensional array. Forexample, the two-dimensional array can have eight rows corresponding tothe eight uniform test patches 575 and 17,280 columns corresponding tothe set of printable cross-track pixel locations. A singular valuedecomposition algorithm can then be used to decompose thetwo-dimensional array into a set of one-dimensional feature vectors.Singular value decomposition algorithms are well-known in the art andare available in a variety of different commercially-available dataanalysis software packages.

In an exemplary embodiment, the singular value decomposition isdetermined on an N_(x)×N_(p) two-dimensional array where N_(x) is thenumber of printable cross-track pixel locations (e.g., N_(x)=17,280) andN_(p) is the number of uniform test patches 575 (e.g., N_(p)=8). Theresult is a set of N_(x)×1 cross-track feature vectors 820 and a set of1×N_(p) pixel-value feature vectors 825 An interpolation process (e.g.,a spline interpolation) is then used to determine a set of 1×N_(v)pixel-value feature vectors 825 that spans the full range of inputtone-levels, where N_(v) is the number of input tone-levels (e.g.,N_(v)=4096). Preferably the interpolation process is constrained suchthat the pixel-value feature vectors 825 are zero at the minimum andmaximum tone-levels (e.g., v=0 and 4095). In other embodiments, the datafor the tone-level correction function 805 determined for the set ofuniform test patches 575 can be interpolated to determine an N_(x)×N_(v)two-dimensional array covering the full set of possible tone-levels asdiscussed earlier. In this case the singular value decomposition can beperformed on the full N_(x)×N_(v) array such that no interpolation isrequired on the determined pixel-value feature vectors 825.

FIG. 19 shows a set of cross-track feature vectors 820 (i_(f)(x)) andpixel-value feature vectors 825 (k_(f)(v)) for an exemplary singularvalue decomposition of the tone-level correction function 805 shown inFIG. 18. In this example, the SVD function in the commercially-availableMatlab software application was used to perform the singular valuedecomposition with the total number of feature vectors being set to n=3.In other examples, different numbers of features vectors could be used,for example the numbers of features vectors could be in the range1≤n≤10, or more typically in the range 2≤n≤4. The number of featurevectors that are appropriate for a particular application will depend onthe non-uniformity characteristics of the particular printer.

It has been found that two-dimensional tone-level correction functions805 of the form Δ_(c)(x,v) are adequate to represent the requiredcorrections for many printing systems where the predominant tone-levelerrors vary as a function of cross-track position and tone-level.However, in some cases the tone-level errors may also vary with in-trackposition (e.g., due to contributing factors such as roller imperfectionsthat can impact in-track motion). In this case, the tone-levelcorrection function 805 can be generalized to have the formΔ_(c)(x,y,v), and the corresponding vector decomposition can begeneralized to have the form:

$\begin{matrix}{{\Delta_{c}( {x,y,v} )} \cong {\sum\limits_{f = 1}^{n}{{i_{f}(x)}{{oj}_{f}(y)}{{ok}_{f}(v)}}}} & (14)\end{matrix}$where j_(f)(y) are one-dimensional feature vectors taken along thein-track axis. In this case, the corrected image data can be determinedusing a generalization of the relationship of Eq. (12):v _(c)(x,y)=v(x,y)+Δ_(c)(x,y,v(x,y))  (15)

In the preceding examples, the tone-level correction function 805Δ_(c)(x,y,v(x,y)) did not include any time dependence. More generally,there can be a time dependence so that the tone-level correctionfunction 805 has the form Δ_(c)(x,y,v(x,y),t). For cases where thesimplifying assumption of time independence is appropriate, thefollowing condition will be true:Δ_(c)(x,y,v(x,y),t=t ₁)≈Δ_(c)(x,y,v(x,y),t=t ₂),∀t ₁ ,t ₂≥0  (17)While this assumption is often approximately accurate when the timedifference |t₂−t₁| is relatively small, as the digital printingtechnologies continue being challenged to maximize the time between twoconsecutive operator intervention with the ultimate objective ofautonomous printing, this assumption will no longer be valid.

For cases where the tone-level correction function 805 has a timedependence, the corresponding vector decomposition can be generalized tohave the form:

$\begin{matrix}{{\Delta_{c}( {x,y,v,t} )} \cong {\sum\limits_{f = 1}^{n}{{i_{f}(x)}o\;{j_{f}(y)}o\;{k_{f}(v)}o\;{l_{f}(t)}}}} & (18)\end{matrix}$where l_(f)(t) are one-dimensional temporal feature vectors. In thiscase, the corrected image data can be determined using the followingrelationship:v _(c)(x,y,t)=v(x,y,t)+Δ_(c)(x,y,v(x,y),t)  (19)where t is the time that the (x,y) pixels are printed.

The temporal feature vectors l_(f)(t) capture variations associates witha number of different perturbation sources in the printing system. Forexample, various rotating components (e.g., imaging member 111,intermediate transfer member 112, transfer backup member 113 anddevelopment station 225 in FIG. 2) can introduce periodic perturbationsinto the printing subsystem 31 (FIG. 2) that can result in correspondingperiodic variations in the printed image. The periodic variationsassociated with a particular perturbation source will have a period Tbeing the circumference of the rotating component divided by itsrotational speed. Temporal perturbations can also be introduced fromother types of perturbation sources, for example when a piece ofreceiver 42 enters the second transfer nip 202 it will introduce a shockwave into the printing subsystem 31 that can cause variations in theprinted image. Each of the perturbation sources will have a startingtime associated with the perturbation. For example, the time that thereceiver 42 enters the second transfer nip 202 or the time when arotating component is in a particular predefined orientation. Thetemporal feature vectors l_(f)(t) can be decomposed into a summation ofterms associated with each of the perturbation sources using arelationship of the form:

$\begin{matrix}{{l_{f}(t)} = {\sum\limits_{m = 1}^{M}{l_{f}^{m}( {t - t_{0}^{m}} )}}} & (20)\end{matrix}$where l_(f) ^(m)(t) is the feature vector for the m^(th) perturbationsource that introduces a temporal variation, and t₀ ^(m) is the startingtime for that perturbation source. The temporal variations associatedwith each perturbation source will have a corresponding period T^(m). Aconstrained optimization process is used to determine the decompositionof the temporal feature vector l_(f)(t) into the feature vectors l_(f)^(m)(t) for each perturbation source based on the known periodicityassociated with each perturbation source.

The tone-level correction function 805 discussed relative to Eqs.(18)-(19) include four individual one-dimensional feature vectorsi_(f)(x), j_(f)(y), k_(f)(v), l_(f)(t) which vary in cross-trackposition, in-track position, pixel value and time, respectively. In somecases, only a subset of these feature vectors are required to adequatelyrepresent the tone-level correction function 805. For example, in somecases it may be acceptable to neglect the in-track variations to give atone-level correction function 805 of the form:

$\begin{matrix}{{\Delta_{c}( {x,v,t} )} \cong {\sum\limits_{f = 1}^{n}{{i_{f}(x)}o\;{k_{f}(v)}{{ol}_{f}(t)}}}} & (21)\end{matrix}$where i_(f)(x) are one-dimensional cross-track feature vectors, k_(f)(v)are one-dimensional pixel value feature vectors, and l_(f)(t) areone-dimensional temporal feature vectors. In this case, the correctedimage data can be determined using the following relationship:v _(c)(x,y,t)=v(x,y,t)+Δ_(c)(x,v(x,y),t)  (22)

A flowchart of an exemplary process that can be used to modify the imagepixels of an input digital image using the tone-level correctionfunction 805 according to an exemplary embodiment is shown in FIG. 20.Each input image pixel 960 of the input digital image is processed byusing a determine tone-level correction step 965 to determine atone-level correction value 970 responsive to the tone-level correctionfunction 805. In an exemplary embodiment, the tone-level correctionfunction 805 is represented as a vector decomposition and the determinetone-level correction step 965 determines the tone-level correctionvalue 970 using the relationship given in Eq. (18).

A modify image pixel step 975 is then used to determine a modified imagepixel 980. In an exemplary embodiment, the modify image pixel step 975determines the modified image pixel 980 by adding the tone-levelcorrection value 970 to the pixel value of the input image pixel 960 asshown in Eq. (19). It will be obvious to one skilled in the art that inother embodiments other forms of modification functions could be used.For example, the tone-level correction function 805 can store scalevalues rather than difference values. In this case the tone-levelcorrection value 970 would be a scale factor such that the modifiedimage pixel 980 can be determined by multiplying the pixel value of theinput image pixel 960 by tone-level correction value 970. In otherembodiments, the tone-level correction function 805 can store themodified image values themselves rather than difference values that areadded to the input pixel values to determine the modified image values.In this case, the determine tone-level correction step 965 and themodify image pixel step 975 are effectively combined into a singleoperation.

FIG. 21 is a high-level diagram showing the components of a system forprocessing image data according to embodiments of the present invention.The system includes a data processing system 710, a peripheral system720, a user interface system 730, and a data storage system 740. Theperipheral system 720, the user interface system 730 and the datastorage system 740 are communicatively connected to the data processingsystem 710.

The data processing system 710 includes one or more data processingdevices that implement the processes of the various embodiments of thepresent invention, including the example processes described herein. Thephrases “data processing device” or “data processor” are intended toinclude any data processing device, such as a central processing unit(“CPU”), a desktop computer, a laptop computer, a mainframe computer, atablet computer, a digital camera, cellular phone, or any other devicefor processing data, managing data, or handling data, whetherimplemented with electrical, magnetic, optical, biological components,or otherwise. In some embodiments, the data processing system 710includes a plurality of data processing devices distributed throughoutvarious components of the printing system (e.g., the pre-processingsystem 305 and the print engine 370).

The data storage system 740 includes one or more processor-accessiblememories configured to store information, including the informationneeded to execute the processes of the various embodiments of thepresent invention, including the example processes described herein. Thedata storage system 740 may be a distributed processor-accessible memorysystem including multiple processor-accessible memories communicativelyconnected to the data processing system 710 via a plurality of computersor devices. On the other hand, the data storage system 740 need not be adistributed processor-accessible memory system and, consequently, mayinclude one or more processor-accessible memories located within asingle data processor or device.

The phrase “processor-accessible memory” is intended to include anyprocessor-accessible data storage device, whether volatile ornonvolatile, electronic, magnetic, optical, or otherwise, including butnot limited to, registers, floppy disks, hard disks, Compact Discs,DVDs, flash memories, ROMs, and RAMs.

The phrase “communicatively connected” is intended to include any typeof connection, whether wired or wireless, between devices, dataprocessors, or programs in which data may be communicated. The phrase“communicatively connected” is intended to include a connection betweendevices or programs within a single data processor, a connection betweendevices or programs located in different data processors, and aconnection between devices not located in data processors at all. Inthis regard, although the data storage system 740 is shown separatelyfrom the data processing system 710, one skilled in the art willappreciate that the data storage system 740 may be stored completely orpartially within the data processing system 710. Further in this regard,although the peripheral system 720 and the user interface system 730 areshown separately from the data processing system 710, one skilled in theart will appreciate that one or both of such systems may be storedcompletely or partially within the data processing system 710.

The peripheral system 720 may include one or more devices configured toprovide digital content records to the data processing system 710. Forexample, the peripheral system 720 may include digital still cameras,digital video cameras, cellular phones, or other data processors. Thedata processing system 710, upon receipt of digital content records froma device in the peripheral system 720, may store such digital contentrecords in the data storage system 740.

The user interface system 730 may include a mouse, a keyboard, anothercomputer, or any device or combination of devices from which data isinput to the data processing system 710. In this regard, although theperipheral system 720 is shown separately from the user interface system730, the peripheral system 720 may be included as part of the userinterface system 730.

The user interface system 730 also may include a display device, aprocessor-accessible memory, or any device or combination of devices towhich data is output by the data processing system 710. In this regard,if the user interface system 730 includes a processor-accessible memory,such memory may be part of the data storage system 740 even though theuser interface system 730 and the data storage system 740 are shownseparately in FIG. 21.

A computer program product for performing aspects of the presentinvention can include one or more non-transitory, tangible, computerreadable storage medium, for example; magnetic storage media such asmagnetic disk (such as a floppy disk) or magnetic tape; optical storagemedia such as optical disk, optical tape, or machine readable bar code;solid-state electronic storage devices such as random access memory(RAM), or read-only memory (ROM); or any other physical device or mediaemployed to store a computer program having instructions for controllingone or more computers to practice the method according to the presentinvention.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations, combinations, and modifications can be effected by a personof ordinary skill in the art within the spirit and scope of theinvention.

PARTS LIST

-   31 printing subsystem-   32 printing subsystem-   33 printing subsystem-   34 printing subsystem-   35 printing subsystem-   38 print image-   39 fused image-   40 supply unit-   12 receiver-   42 a receiver-   42 b receiver-   50 transfer subsystem-   60 fuser module-   62 fusing roller-   64 pressure roller-   66 fusing nip-   68 release fluid application substation-   69 output tray-   70 finisher-   81 transport web-   86 cleaning station-   99 logic and control unit (LCU)-   100 printer-   111 imaging member-   112 intermediate transfer member-   113 transfer backup member-   201 first transfer nip-   202 second transfer nip-   206 photoreceptor-   210 charging subsystem-   211 meter-   212 meter-   213 grid-   216 surface-   220 exposure subsystem-   225 development station-   226 toning shell-   227 magnetic core-   240 power source-   300 page description file-   305 pre-processing system-   310 digital front end (DFE)-   315 raster image processor (RIP)-   320 color transform processor-   325 compression processor-   330 image processing module-   335 decompression processor-   340 halftone processor-   345 image enhancement processor-   350 image data-   360 metadata-   370 print engine-   400 print engine-   405 data interface-   410 metadata interpreter-   415 control signals-   416 resolution modification flag-   417 resize factor-   418 halftoning flag-   419 halftoning parameters-   420 resolution modification processor-   421 modify resolution test-   422 resolution modification operation-   425 halftone processor-   426 halftone image test-   427 halftoning operation-   428 processed image data-   430 printer module controller-   435 printer module-   450 printed image-   500 input pixel-   505 define high-resolution printer coordinates step-   510 high-resolution printer coordinates-   515 determine high-resolution dot coordinates step-   520 high-resolution dot coordinates-   525 determine high-resolution halftoned pixel values step-   530 halftone dot function-   535 high-resolution halftoned pixel values-   540 average high-resolution halftoned pixel values step-   545 halftoned pixel-   550 more pixels test-   570 alignment marks-   575 uniform test patches-   580 first color channel image content-   581 second color channel image content-   582 third color channel image content-   583 fourth color channel image content-   590 cross-track direction-   595 in-track direction-   600 dot shape function-   605 threshold value function-   610 edge softness function-   615 edge softness function-   620 halftoned image-   625 halftoned image-   710 data processing system-   720 peripheral system-   730 user interface system-   740 data storage system-   800 resolution/tone-level processor-   805 tone-level correction function-   810 tone-level correction operation-   820 cross-track feature vectors-   825 pixel-value feature vectors-   900 test target-   905 print test target step-   910 printed test target-   915 capture image step-   920 captured image-   925 analyze captured image-   930 measured tone-level characteristics-   935 nominal tone level characteristics-   940 determine tone-level error characteristics step-   945 tone-level error characteristics-   950 determine tone-level correction function step-   955 scanner calibration function-   960 input image pixel-   965 determine tone-level correction step-   970 tone-level correction value-   975 modify image pixel step-   980 modified image pixel

The invention claimed is:
 1. A method for correcting tone-levelnon-uniformities in a digital printing system, comprising: a) providingdigital image data for a test target including a set of uniform testpatches, each test patch having an associated pixel value and extendingin a cross-track direction; b) printing the test target using thedigital printing system to provide a printed test target; c) using adigital image capture system to capture an image of the printed testtarget; d) using a data processing system to automatically analyze thecaptured image to determine a measured tone-level as a function of across-track position for each of the test patches; e) comparing themeasured tone-levels to nominal tone-levels for each test patch todetermine measured tone-level errors as a function of the cross-trackposition for each of the test patches; f) determining a tone-levelcorrection function responsive to the measured tone-level errors,wherein the tone-level correction function specifies tone-levelcorrections to be applied as a function of cross-track position, pixelvalue and time; g) storing a representation of the tone-level correctionfunction in a digital memory, wherein the representation of thetone-level correction function includes a set of one-dimensional featurevectors corresponding to a decomposition of the tone-level correctionfunction into a summation of outer vector products of theone-dimensional feature vectors; h) receiving digital image data for aninput digital image to be printed by the digital imaging system, whereinthe digital image data specifies input pixel values for an array ofimage pixels; and i) determining corrected image data by: using thestored representation of the tone-level correction function to determinea tone-level correction value for each image pixel responsive to theinput pixel value of the image pixel and the cross-track position of theimage pixel; and modifying the input pixel value for each image pixelresponsive to the determined tone-level correction value to provide acorrected pixel value; wherein the decomposition of the tone-levelcorrection function has the form:${\Delta_{c}( {x,v,t} )} \cong {\sum\limits_{f = 1}^{n}{{i_{f}(x)}o\;{k_{f}(v)}{{ol}_{f}(t)}}}$wherein Δ_(c)(x,v,t) is the tone-level correction function, i_(f)(x) areone-dimensional cross-track feature vectors taken along the cross-trackaxis of the tone-level correction function, k_(f)(v) are one-dimensionalpixel value feature vectors taken along the pixel value axis of thetone-level correction function, l_(f)(t) are one-dimensional temporalfeature vectors taken along the time axis of the tone-level correctionfunction, x is the cross-track position, v is the pixel value, t is thetime value, f is a feature vector index, n is the total number offeature vectors, and ∘ is the outer vector product operator.
 2. Themethod of claim 1, wherein the decomposition of the tone-levelcorrection function is determined using a singular value decompositionalgorithm.
 3. The method of claim 1, wherein the one-dimensionaltemporal feature vectors are further decomposed into a summation oftemporal feature vectors corresponding to individual perturbationsources:${l_{f}(t)} = {\sum\limits_{m = 1}^{M}{l_{f}^{m}( {t - t_{0}^{m}} )}}$where M is the total number of perturbation sources, l_(f) ^(m)(t) isthe temporal feature vector for an m^(th) perturbation source, and t₀^(m) is a starting time associated with the m^(th) perturbation source.4. The method of claim 3, wherein the perturbation sources include oneor more rotating components, and wherein the starting time t₀ ^(m) isthe time when the rotating component is in a predefined orientation. 5.The method of claim 3, wherein the perturbation sources include a shockwave associated with a piece of receiver entering a nip, and wherein thestarting time t₀ ^(m) is the time when the receiver enters the nip. 6.The method of claim 1, wherein the total number of feature vectors is inthe range 2≤n≤4.
 7. The method of claim 1, wherein the tone-levelcorrection function is also a function of in-track position pixel. 8.The method of claim 7, wherein the decomposition of the tone-levelcorrection function further includes one-dimensional feature vectorsj_(f)(y) taken along the in-track axis y such that the decomposition ofthe tone-level correction function Δ_(c)(x,y,v,t) has the form:${\Delta_{c}( {x,y,v,t} )} \cong {\sum\limits_{f = 1}^{n}{{i_{f}(x)}o\;{j_{f}(y)}o\;{k_{f}(v)}{{{ol}_{f}(t)}.}}}$9. The method of claim 1, wherein the input pixel values of the inputdigital image have an input bit depth, and further including convertingthe input pixel values of the input digital image to a higher bit depthprior to the step of determining the corrected image data.
 10. Themethod of claim 9, wherein the higher bit depth is at least 10bits/pixel.
 11. The method of claim 9, wherein the higher bit depth isat least 12 bits/pixel.
 12. The method of claim 1, wherein the correctedpixel value is determined by adding the determined tone-level correctionvalue to the input pixel value.
 13. The method of claim 1, wherein thestep of printing the corrected image data includes applying acomputational halftoning algorithm to the corrected image data todetermine halftoned image data.
 14. The method of claim 1, furtherincluding the step of printing the corrected image data using thedigital printing system to provide a printed image with reducedtone-level errors.
 15. A method for correcting tone-levelnon-uniformities in a digital printing system, comprising: a) providingdigital image data for a test target including a set of uniform testpatches, each test patch having an associated pixel value and extendingin a cross-track direction; b) printing the test target using thedigital printing system to provide a printed test target; c) using adigital image capture system to capture an image of the printed testtarget; d) using a data processing system to automatically analyze thecaptured image to determine a measured tone-level as a function of across-track position for each of the test patches; e) comparing themeasured tone-levels to nominal tone-levels for each test patch todetermine measured tone-level errors as a function of the cross-trackposition for each of the test patches; f) determining a tone-levelcorrection function responsive to the measured tone-level errors,wherein the tone-level correction function specifies tone-levelcorrections to be applied as a function of cross-track position, pixelvalue and time; and g) storing a representation of the tone-levelcorrection function in a digital memory, wherein the representation ofthe tone-level correction function includes a set of one-dimensionalfeature vectors corresponding to a decomposition of the tone-levelcorrection function into a summation of outer vector products of theone-dimensional feature vectors; wherein the decomposition of thetone-level correction function has the form:${\Delta_{c}( {x,v,t} )} \cong {\sum\limits_{f = 1}^{n}{{i_{f}(x)}o\;{k_{f}(v)}o\;{l_{f}(t)}}}$wherein Δ_(c)(x,v,t) is the tone-level correction function, i_(f)(x) areone-dimensional cross-track feature vectors taken along the cross-trackaxis of the tone-level correction function, k_(f)(v) are one-dimensionalpixel value feature vectors taken along the pixel value axis of thetone-level correction function, l_(f)(t) are one-dimensional temporalfeature vectors taken along the time axis of the tone-level correctionfunction, x is the cross-track position, v is the pixel value, t is thetime value, f is a feature vector index, n is the total number offeature vectors, and ∘ is the outer vector product operator.